Limitations of liver biopsy have led to the development of various non-invasive evaluations of liver fibrosis that are more suitable for screening, treatment monitoring, and follow-up. Ultrasound shear wave elastography technique is known as technique for non-invasive liver fibrosis staging due to its absolute stiffness quantification capability, real-time, cost-efficient and portable features. Commercial products are already available, for example the Philips Ultrasound new release shear wave Elastography Point Quantification (ElastPQ). With such a method, acoustic radiation force is used to stress the liver or any other anatomical site mechanically and produce a shear wave. The resulting tissue displacement is measured and used to estimate the elasticity of the anatomical site, which for example has been found to correlate with fibrosis stage in the case of a liver being the anatomical site.
Due to system limitations such as transducer heating and physical limitations such as shear wave fast attenuation, current commercial shear wave elastography products usually only provide measurement at a user selected point location or a bigger region of interest (ROI) spatially confined within the B-mode field of view. In a typical workflow, the user selects a suspicious region under the conventional ultrasound B-mode imaging, activates shear wave elastography tool, makes measurements, and repeats the process at another user selected position. A measurement made in the ROI in the imaging plane may then be displayed as a value associated with the ROI and be reported in the unit of Young's modulus, for example on a display of the ultrasound elastography system. For example, by this, a liver stiffness value associated with the ROI may be reported in the unit of Young's modulus. By moving the ROI, a user may inspect a liver in a non-invasive manner.
Document US 2011/0066030 A1 shows an example for an ultrasound imaging system providing dynamic control of a shear wave front used to image viscoelasticity in a biological tissue. The system receives an indication of a region of interest and selects a shear wave front shape. The system also selects, based on the selected shear wave front shape, focus locations for a plurality of push pulses and a sequence for moving a shear wave source among the focus locations. The system transmits a series of push pulses according to the selected sequence, and determines a speed of the shear wave front as it passes through the region of interest. Changes in the speed of the shear wave front are related to changes in stiffness within the tissue.
Improper positioning of the region of interest could, however, lead to sub-optimal elastography measurements, in particular for liver fibrosis staging. There are several criteria that go into making the elastography measurement an optimal measurement. For the application of liver fibrosis staging, for example, recommended scan protocols by manufactures usually suggest placing the ROI in a region without and away from artifacts. Elastography measurement results can suffer from the poor placement of the ROI for two reasons. First, shear waves reflected by artifact wall in the lateral direction may contaminate stiffness reconstruction if filtering on reflection is not efficient. Second, the ROI may be placed right behind or in front of an artifact in the depth direction. Strong specular reflection caused by the artifact wall will reduce push pulse energy leading to a lower shear wave signal-to-noise-ratio and unstable stiffness reconstruction.
There is a need to further improve such elastography system.